1. Field of the Invention
The present invention is related to a reciprocating pump capable of miniaturization for use in a medication dosage device, and in particular to such a pump which passes, i.e., is not blocked or stopped by gas bubbles which may be present in the liquid medication.
2. Description of the Prior Art
For reasons of miniaturization, it is preferable to use a reciprocating pump as the pump unit in implantable medication dosage devices such as, for example, an insulin dosage device. Such devices convey a liquid medication at low rates for delivery to a patient. The piston displacement of such a pump required for this type of use is approximately 0.5 through 1.0 microliter.
It is also preferable that such a reciprocating pump be capable of pumping gas bubbles with a satisfactory conveying rate, so that delivery of the medication does not cease given the presence of a gas bubble at some location in the fluid passage of the device, so that the patient does not remain unsupplied with medication over a longer time. Gas bubbles in the medication reservoir may arise, for example, due to gas generation within the medication, or during refilling of the medication reservoir.
The problem of gas bubble conveying or pumping is even more pronounced in a medication dosage device wherein, as is typical, the medication reservoir is charged with an underpressure, i.e., a pressure below atmospheric pressure, as in the device described in German OS No. 26 52 026. In a typical case, such underpressure in the medication reservoir, which is advantageous for safety reasons, is between 0.5 and 1.0 bar. Given an ideal pump, having a pump chamber free of dead space, the gas conveying rate may be smaller than the liquid conveying rate by a factor as large as two.
If, by contrast, the pump has dead space in the pump chamber, the dead space being the same size as the displacement volume, conveying of gas bubbles is impossible at the lower limit of the underpressure (0.5 bar). A gas bubble situated in the pump chamber under such conditions would be merely compressed and decompressed, without being transported to the output side of the device. Reliable functioning of the device is thus not guaranteed given lower underpressures. This problem usually does not arise, however, in larger pumps having a displacement volume greater than 10 .mu.liters.
The above problem becomes acute, however, in devices which are typical of many medication dosage devices, wherein the medication reservoir is at an underpressure of 300 through 500 mbar (500-700 mbar absolute), as is used in many insulin infusion devices.
The same problem arises when pumping or conveying is undertaken against an overpressure, for example, against a substantially plugged catheter. In general terms, if there is a significant difference between the (low) input pressure and the (high) output pressure at the pump, the risk is present that no medication will be conveyed, even if the piston motion is maintained. As discussed above, the pressure difference may arise due to underpressure in the medication reservoir, or overpressure at the catheter. For many of the above reasons, it is preferable to have a ratio which is as large as possible, given other construction constraints, between the displacement volume and the dead space in the pump chamber of a reciprocating pump. This ratio is referred to as the compression ratio. Various miniaturized pumping systems have been devised by those skilled in the art in an effort to improve the compression ratio.
A reciprocating pump is disclosed, for example, in U.S. Pat. No. 4,568,250 having an input chamber formed by an end face of a housing, a cylindrical wall, and the end face of a piston. The end face of the housing has an opening connected to a reservoir. A moveable element of a check valve is disposed inside the input chamber. A mechanical spring system holds the moveable part pressed against the input opening in a rest position. This structure therefore has a relatively large dead space, having an unfavorable influence on the gas conveying capability of this system, due to the spring system and its associated fastening means.
Another reciprocating pump is described in U.S. Pat. No. 3,468,257 having a check valve disposed at the output side of the pump. This valve is also actuated by a mechanical spring system, and requires a relatively large dead space for the fluid. This structure therefore presents the same problems as discussed above in the event of the appearance of gas bubbles in the fluid to be conveyed.
German OS No. 35 15 848 discloses a valve for medication dosage devices. This valve is not used as a component part of such devices, but as an additional element which is placed in the intake line or the discharge line associated with the dosage device. In this valve, the closing force of the moveable element is magnetically generated. The moveable element may be spherical, and presses against a resilient O-ring. This valve, however, does not result in a tight placement of the moveable element against the piston of the dosage device, and would not do so even if this valve structure were to be integrated within a medication dosage device. Therefore even in the case of such integration, a relatively large dead space would also arise, thereby still presenting the same problems with respect to gas bubble pumping as discussed above.
An embodiment as descirbed in FIG. 3 of European Application No. 0 103 536 showing a reciprocating pump with an integrated check valve. The check valve is disposed at the output side of the pump. This pump has a piston moveable in a cylinder. There is no sealing closure between the piston and the cylinder wall, so that movement of the piston in a first direction displaces the medium to be conveyed in an output direction, and the device is simultaneously charged with fresh medium from a reservoir connected thereto. The piston speed is selected such that the medium does not entirely flow through the gap between the piston and the cylinder wall given the presence of an opposing pressure. When the piston moves in a second, opposite direction, the pump chamber is again filled by medium flowing through the gap between the piston and the cylinder wall. The check valve is formed by a resilient membrane which terminates a longer channel. Again, difficulties and the potential for malfunction arise if gas bubbles are contained in the medium.